To troubleshoot a problem, you need to know what is normal.
To establish a baseline, collect data from your system under normal
circumstances. Collect this data over long periods of time. Performance logs are
an easy way of establishing a baseline. The easiest way to create a baseline is
to collect data with System Monitor into a
log file. The log files can be analyzed with
System Monitor or a spreadsheet program such as Excel.

Baseline information can be used for;

Capacity planning

Use trend analysis to predict future usage

Optimization

Determine causes of performance changes

Troubleshooting

When problems occur, you have a reference point for
normal behavior

At a minimum, baselines must have details on the basic
resources. The following table shows the most basic objects to monitor.

Device

Object

Counter

Processor

Processor

% processor time

Memory

Memory

Pages/sec

Disk

Physical disk

Avg. disk queue length

Network

Network Segment

% network utilization

The following graph shows a baseline which is
typical for any type of computer resource utilization. It is low at night, high
during the day and dips slightly at lunch time. This shows an actual situation
that I experienced where problems appeared during lunch time. Instead of the
normal lower utilization, we observed extremely high utilization. Further
investigation showed lunch break game playing which was subsequently terminated.
Problem solved!